amd64: eliminate td_md.md_fpu_scratch

For signal send, copyout from the user FPU save area directly.

For sigreturn, we are in sleepable context and can do temporal
allocation of the transient save area. We cannot copying from userspace
directly to user save area because XSAVE state needs to be validated,
also partial copyins can corrupt it.

Requested by: jhb
Reviewed by: jhb, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31954